Sample introduction system for mass spectrometer analysis



March 10, 1970 F. G. PADRTA 3,500,040

SAMPLE INTRODUCTION SYSTEM FOR MASS SPECTROMETER ANALYSIS Filed Sept.28, 1967 F lgure *z i %I 0 Q 2 Ema/4s! Gas Line" i ,-Ana/ysis Unit /5 7l2 g /4- up f L 6 Adjustable 'B-Par/ Valve 9 1 g l3" 2 i X K To VacuumPump Tube Filling Figure 2 r 24 4 Compressor R/ngs IN VEN $45 4 Frank 6.Padrfa I 23 7 B 25 I Tub/n9 A TTORNEYS United States Patent US. Cl.25041.9 5 Claims ABSTRACT OF THE DISCLOSURE A sample introducing systemfor a mass spectrometer utilizing a short length of tubing with amolecular leak inlet section of a thin diaphragm having central openingmeans of the order of about .004" in diameter and an adjustable splittervalve member providing two outlet means from the tubing. One outlet isadapted to connect with a vacuum pump and the other to cause afractional portion of the withdrawn gaseous sample stream to be shuntedto the mass spectrometer.

This application is a continuation-in-part of my previously filedapplication Ser. No. 654,773, filed July 20, 1967, which is nowabandoned.

The present invention relates to an improved arrangement for effectingthe rapid introduction of a gaseous sample into a mass spectrometeranalysis unit. More particularly, the invention is directed to a specialtubular sample inlet system which, in part, utilizes an orifice typemolecular leak means as an inlet section and a stream splitting gasdistribution means that cooperates in providing a continuous fasttransfer of a sample stream to the analysis unit.

The usual types of sample withdrawal means for a mass spectrometer,whether periodic or continuous, utilize a capillary tube inlet sectionso that merely a small portion of the material to be analyzed from aprocess or product stream will be withdrawn for transfer to theparticular analysis unit. However, the capillary tube arrangementresultsin a slow, large surface area transfer passageway such that a portion ofthe stream, or condensate therefrom may be adsorbed on the wall of thetubing. Alternatively, highly reactive components can undergo undesiredreactions with other accompanying constituents during the time periodrequired to effect the transfer through the capillary tubing. Forinstance, in attempting to carry out the continuous sampling of an autoengine exhaust stream for the monitoring of entrained nitric oxide (NO)in a mass spectrometer unit, it is necessary to obtain a fast samplingoperation to minimize adsorption of entrained hydrocarbons'to the tubewall, as well as necessary to minimize the reaction of the nitric oxidewith oxygen and other constituents in the stream. Preferably thesampling is to be attained in a period of time of less than about 3seconds. Also, it may be pointed out that an automotive exhaust streamis continually changing with varying operating speeds and conditions andrather continuous sampling is needed to obtain a proper analysis of anyone component in such an exhaust stream.

This usual mass spectrometer, including those for nitric oxidemonitoring, provides a vacuum for the sample intake so that, for thepresent type of sample introduction system used with auto exhaust gases,there isv a pressure differential involved of from about 1 atmosphere inthe exhaust line to a vacuum of about 10* millimeters of mercury asprovided by the analysis unit. However, since the gaseous flow in theauto exhaust line may vary in pressure, a preferred system provides fora vacuum pump connectlon to bring the sample stream to the zone of theanalyzer at uniform conditions.

Along with the use of capillary tube type inlet means for usual samplingsystems, there have been used glass frit discs, and the like, directlyahead of the analysis unit so as to limit or proportion the samplestream flow to such unit. However, systems using frits have noadjustability and tend to have operating difliculties because of dirtaccumulations, particularly with auto exhaust gas streams which canprovide carbon particles and condensation of hydrocarbons. Also, it hasbeen found that certain frits, such as of sintered silicon carbide maytend toinitiate or enter into a chemical reaction with certain of theexhaust constituents and thus provide still further dilliculties inobtaining accurate analysis for nitric oxide or for any other particularcomponent.

It may be considered a principal object of the present invention toutilize a gas sample introduction system for an analysis unit whichprovides for rapid withdrawal and shunting of a sample portion of agaseous stream to the analyzer.

It may be considered a further object of this invention to provide acompact sample introduction system particularly adapted to effect themonitoring of the NO content in an exhaust gas stream by eliminatinglong tubular means and any materials which tend to enter into reactionwith such gas stream.

In a broad aspect, the present invention provides a sample introductionsystem for a mass spectrometer which is particularly adapted to effectrapid sampling of a gaseous stream and preclude reaction of an entrainedhighly reactive material with other accompanying constituents, whichcomprises in combination, a molecular leak inlet section connective witha sample receiving tube, with such leak section comprising a thindiaphragm mem ber havingat leastone small opening, with open area beingless than about 7.0 10- square inches, and an adjustable outlet meansfrom said sample tube providing two outlet passageways therefrom,whereby a vacuum source may pull a samplestream to the zone of saidoutlet passageways and then a fractional portion shunted therefrom tosaid mass spectrometer.

A preferred design utilizes a thin foil membrane, as of gold, for thethin diaphragm member having the orifice opening to provide in effectingmolecular leak. Actually, theenti're introduction system is preferablyof glass or stainless steel or other smooth substantially nonactive typeof material such that there will be a minimum of adsorption or retentionof particulates or of any entrained gaseous components passing throughthe system. Also, as will be set forth more fully hereinafter, theoutlet passageway means preferably utilizes an adjustable three-portvalve where there is a means for accurately adjusting the outlet flowfrom at least one of the outlet ports. Generally, in a conventionalsplitter valve, or three-port valve, there will be a straight throughpassageway for continuous flow of at least'a' portion of the fluidstream and an adjustable tapered plug in a needle valve design fromthesecond of the outlet ports of the valve unit.

I Reference to the accompanying drawing and the following descriptionthereof will serve to more clearly illustrate one embodiment of theimproved sample intro duction system for amass spectrometer unit, aswell as point out additional advantageous features which are ob tainedthrough the use of such specially constructed system.

DESCRIPTION OF THE DRAWING FIGURE 1 of the drawing is a diagrammaticelevational type view indicating the arrangement for the rapid transferof a sample stream through the improved introduction system to ananalysis unit.

FIGURE 2 of the drawing indicates in a sectional view one means forattaching a thin gold foil membrane to the end of a tubular member so asto provide a molecular leak inlet section thereto.

FIGURE 3 of the drawing illustrates another form of molecular leak inletsection constructed of glass and adopted for direct insertion into aglass line carrying the gaseous stream to be analyzed.

Referring now particularly to FIGURE 1 of the drawing, there is shown asample withdrawal tube 1 connecting to and projecting slightlyinteriorly of a line 2 which is carrying a stream that is to be analyzedfor at least one entrained gaseous component. The inlet end of thesample withdrawal tube 1 is provided with a thin diaphragm member 3having a small orifice opening 4 and clamp means 5. The latter is merelyindicated diagrammatically and may constitute any suitable holding orclamping means that is capable of attaching the diaphragm member 3 tothe end of the tube member 1. The diaphragm 3 is preferably very thinand in the nature of gold foil or other equivalent noble metal foil,whereby the depth of the passageway provided by orifice 4 is almostnegligible.

For continuous sampling of a particular gaesous component, as forexample nitric oxide in an automotive exhaust gas stream, the size of asingle orifice will be about .003" in diameter and in the range, of.0008" to .004" whereby only a very small fractional portion of thecontinuously flowing exhaust gas stream will be taken into the sampleintroduction system. With two orifices, each will be about .002" indiameter. With other gas analysis operations the leak may be of aslightly different size and thus be within the range of from about 0.5 lto about 7.0x 10- square inches.

The tubing 1 is preferably kept. relatively short in length, generallyless than about 1 foot long, and preferably only about 3" long whenusing Pyrex glass tubing which is of the order of 5 millimeter insidediameter. As pointed out hereinbefore, preferably the tubing is suppliedof glass, stainless steel, or other smooth and chemically inert type ofmaterial and will be of a small diameter /s" to At least the materialshall be non-reactive with the particular component to be analyzed inthe mass spectrometer or other analyzer unit.

In the present drawing, the tubing 1 is shown connecting with one formof adjustable three-port valve unit 6,

which again is preferably of stainless steel, glass, or the like. Thevalve 6 is provided with a straight through passageway 7 suitable forcarrying gas or fluid from an inlet port section 8 to an outlet port 9and thence into tubing 10, as well as suitable for effecting theshunting of a fractional portion of the stream through a small port 11communicating with an outlet connection 12, in turn carrying to ananalysis unit 13. The adjustable feature of valve 6 provides a turnablevalve stem section 14 connecting between an exterior grip section 15 andan internal needle like valve plug or projection 16. The latter moveswithin the outlet orifice opening 11 and meters the flow therethroughinto the analysis unit 13. Although not shown in the drawing, apreferred form of device will have heating means, such as resistancecoil wrapping, around that portion of the device illustrated by numerals1 through 14, except for tube 10. The heating is needed to preventcondensation of any of the constituents gases flowing to the analysisinstrument.

In the present operation for effecting the rapid withdrawal of a samplegas stream and its introduction into a mass spectrometer type of unit,there will be a vacuum pump connection to line 10 and valve at port 9such that there is a positive withdrawal and flow of a sample of gasthrough the inlet end of the system at the leak section 4. The withdrawngas sample stream is thus continuously carried down into the straightthrough passageway 7 of valve 6 whereby a major portion of a stream willcarry on to the vacuum pump means and a fractional portion shuntedthrough the metered passageway 11 and outlet port 12 to be thenintroduced into analysis unit 13.

Where desired, there may be adjustment of the flow through port 11 bymeans of the moving stem 14 and needle valve means 16. Generally, ananalyzer unit of the mass spectrometer type, will operate under avacuum, such as about 10' millimeters of mercury; however, as notedhereinbefore, where the gas stream being transferred in line 2 is undera low pressure, or at least under varying pressure conditions, there ispreferably used a vacuum pump connection to line 10 in order to insure auniform continuous gas flow down into the flow splitting arrangement fortube 1, as provided by the three-port valve means '6.

In FIGURE 2 of the drawing there is shown the utilization of standardtube fitting means for attaching a diaphragm or thin foil molecular leakmember, such as 17, to the end of a sample withdrawal tube, beingindicated as 18. The arrangement is, of course, of advantage inproviding a simple means for having a removable screw cap member 19effect the holding of an insertable diaphragm or leak member 17. In thiscase, the latter is clamped between the underside of cap member 19 andabove compressor rings 20 and 21, with the latter in turn being designedto fit against the inwardly sloping end of a standard tube connectorfitting 22. The latter, in turn, is shown connecting to the tube 18 bymeans of an internally threaded member 23 which bears against internalcompression ring members 24 and 25 encompassing the end of tubing 18within the end of tube fitting 22.

In FIGURE 3 of the drawing there is shown still another form of inlet ormolecular leak section, where such entire section is of glass and isadapted to be inserted directly into a glass tube line which is carryingat least a portion of the gaseous stream to be analyzed. In other words,the enlarged section 26 has end portions 27 and 28 which can be sealedinto a glass tube gas conducting line, indicated as 29. Connecting tothe side of section 26 is a short glass tube section 30 which extendsinto the interior of the enlarged portion at 31 and holds a transversethin diaphragm or gold foil member 32 in a crimped end portion 33. Thediaphragm or foil member 32, as noted with the other embodiments, shallhave at least one small molecular sized opening to restrict the gas flowto the analyzer zone. For NO analysis, the leak opening will be of theorder of .003" diameter, or two openings of the order of .002" diameterto provide an area of about 0.628 10- square inches.

The glass tube 30 may connect directly to a valve unit, such as 6 inFIGURE 1, or be sealed and joined to a different short length of tubingwhich will have the same coefficient of expansion as glass, such asKovar metal tubing. The latter will then connect to the valve unit. Inany case, the tubing length will be short, with the total being lessthan one foot and preferably only 3" or 4". Also, the entire glasssection and the valve section will be suitably heated, by electricresistance heating, or otherwise, to preclude condensation problems.

It is, of course, not intended to limit the present invention to any onemeans for holding a diaphragm member and small molecular sized orificeleak across the end of a tube in order to provide the inlet to thelatter, inasmuch as various clamping or sealing means will be obvious tothose skilled in the art of working with tubular equipment. It is,however, the principal feature of the present invention to obtain arapid transfer of the withdrawn sample stream into the inlet systemthrough a small sized orifice that has a minimum of depth or length,versus utilizing a long piece of capillary tubing.

In addition, modified means, such as various sized molecular leaks, maybe used for fixed designs in providing the bypassing of a portion of thesample stream into the analyzer unit 13 may be provided other thanthrough the use of an adjustable three-port valve, such as 6. However,the latter means is of particular advantage in giving an adjustablecontrol through a small open orifice and precludes the difiiculties thatare frequently obtained through the use of some other means, such asfrits or other porous types of membranes which may be readily clogged bydirt or condensate. The adjustable feature of the valve means is also ofadvantage, as noted hereinbefore, in obtaining a desired controlled flowinto an analysis unit which is to be continuously monitoring a flowstream.

I claim as my invention:

1. A sample introduction system for a mass spectrometer which isparticularly adapted to effect rapid sampling of a gaseous stream andpreclude reaction of an entrained highly reactive material with otheraccompanying constituents, which comprises in combination, a molecularleak inlet section connective with a sample withdrawal tube as an inletmeans thereto, with said leak member comprising a thin diaphragm havingat least one opening and the open area being less than about 7.0 squareinches, and an adjustable outlet means from said sample tube providingtwo outlet passageways therefrom, whereby a vacuum source may pull asample stream to the zone of said outlet means and then a fractionalportion shunted therefrom to said mass spectrometer.

the tube are of a material which provides substantially smooth interiorpassageway means and is a substantially inert non-reactive type ofmaterial.

5. The system of claim 1 further characterized in that said molecularleak inlet section is connective with a glass tubing as said samplewithdrawal tube and the latter, together with said adjustable outletmeans, is encompassed with electrical resistance heating means.

References Cited UNITED STATES PATENTS 2,583,541 1/1952 Berry 250--41.952,601,097 6/1952 Crawford 25041.95 2,721,270 10/1955 Bennett 250-41952,824,967 2/1958 Kamen 25041.9-5

RALPH G. NILSON, Primary Examiner A. L. BIRCH, Assistant Examiner

